1. Field of the Invention
The present invention relates to an amplifying circuit.
2. Description of the Related Art
When operational amplifiers are used, normally a negative feedback circuit for negatively feeding back apart of an output signal is connected between an inverting input terminal and an output terminal. As a result, the operational amplifiers can be used as inverting amplifier circuits or non-inverting amplifier circuits.
Japanese Patent Application Laid-Open No. 2010-35117 discloses a current feedback amplifier that amplifies an audio signal. The current feedback amplifier in Patent Document 1 has an AC feedback loop and a DC feedback loop independently as feedback circuits. The AC feedback loop negatively feeds back a part of an amplified audio signal.
As a characteristic of this current feedback amplifier, an input impedance of the current feedback amplifier is low in view of the AC feedback loop. For this reason, an operation of the current feedback amplifier is changed according to a configuration of the AC feedback loop. That is, a gain of the current feedback amplifier disadvantageously fluctuates due to an impedance of a negative feedback circuit (AC feedback loop).
Non-Patent Document (“Precision Rail-to-Rail Input and Output Operational Amplifiers OP184/OP284/OP484”, [online], Analog Devices, Inc, [searched on Sep. 10, 2010], FIG. 44, <URL: http://www.analog.com/static/imported-files/data_sheets/OP18 4—284—484.pdf>) discloses an operational amplifier using a differential amplifier circuit at an input stage. The differential amplifier circuit disclosed in Non-Patent Document has two NPN transistors and a constant current source. A non-inverting input terminal is connected to a base of one NPN transistor. An inverting input terminal is connected to a base of the other NPN transistor. Emitters of the two NPN transistors are each connected to the constant current source.
Since the emitters of the two NPN transistors are connected to the constant current source, a collector current of each of the two NPN transistors is limited to a lower current than a constant current supplied from the constant current source. As a result, the collector currents of the transistors cannot be fluctuated more greatly than the constant current according to a change in an input signal. The operation of conventional differential amplifier circuits is disadvantageously limited by a value of the constant current supplied from the constant current source.